Field of the Invention
The present invention relates to an electrode material for a lithium-ion secondary battery, an electrode for a lithium-ion secondary battery, and a lithium-ion secondary battery.
Priority is claimed on Japanese Patent Application No. 2016-063910, filed Mar. 28, 2016, the content of which is incorporated herein by reference.
Description of Related Art
Recently, in the rapid progress of technical development of clean energy, efforts to form earth-friendly environments such as the distribution of petroleum dependency-reduced, zero-emission, and power-saving products have become necessary. Particularly, recently, large-capacity storage batteries supplying electric energy to electric vehicles, large-capacity storage batteries supplying electric energy in the case of emergency or disaster, and secondary batteries supplying electric energy to mobile information devices, mobile information terminals, and the like have been attracting attention. As secondary batteries, for example, lead storage batteries, alkali storage batteries, lithium-ion batteries, and the like are known. Particularly, lithium-ion batteries are capable of achieving size reduction, weight reduction, and higher capacity and, furthermore, have excellent characteristics such as a high output and a high energy density. Due to these facts, lithium-ion secondary batteries have been commercialized as high-output power supplies for electric devices mainly including electric vehicles, and active development is underway throughout the globe regarding materials for next-generation lithium-ion secondary batteries.
In addition, recently, as collaboration of large-capacity storage batteries supplying electric energy and houses, home energy management systems (HEMS) have been attracting attention. HEMES is a system for managing automatic control, the optimization of electric power supply and demand, and the like and cleverly consuming energy by integrating information regarding domestic electricity and control systems such as smart home appliances, electric vehicles, and photovoltaic power generation.
Electrode active materials that are ordinarily used for cathodes of lithium-ion batteries in practical use at the moment are LiCoO2 and LiMnO2. However, Co is not evenly distributed in the Earth and is a rare resource, and this, in a case in which a large amount of Co is used, there is a problem in that the product costs increase and stable supply becomes difficult. Therefore, as alternative cathode active materials of LiCoO2, active research and development is underway regarding cathode active materials such as spinal-based LiMn2O4, ternary LiNi1/3Mn1/3Co1/3O2, lithium iron oxide (LiFeO2), and lithium iron phosphate (LiFePO4). Among these cathode materials, LiFePO4 having an olivine structure is attracting attention as a cathode material that is not only safe but also has no problem from the resource and cost viewpoint. Olivine-based cathode materials represented by LiFePO4 include phosphorus as a constituent element and form a strong covalent bond with oxygen. Therefore, compared with cathode materials such as LiCoO2, the olivine-based cathode material is a material which has no concern of emitting oxygen at a high temperature, also has no concern of a risk of ignition due to the oxidation and decomposition of electrolytic solutions, and has excellent safety.
However, in LiFePO4 having the above-described advantages, there is a problem with poor electron conductivity. This poor electron conductivity is considered to result from the slow diffusion of lithium ions in the active material which is attributed to the structure and low electron conductivity. Therefore, as an electrode material having improved electron conductivity, for example, an electrode material in which multiple primary particles of an electrode active material made of LizAyBzPO4 (A represents one or more selected from the group consisting of Co, Mn, Ni, Fe, Cu, and Cr, B represents one or more selected from the group consisting of Mg, Ca, Sr, Ba, Ti, Zn, B, Al, Ga, In, Si, Ge, Sc, Y, and rare earth elements, 0<x<2, 0<y<1.0, and 0≤z<1.5) gather so as to form secondary particles, carbon is interposed between the primary particles as an electron-conducting substance, and the surface of the electrode active material is coated with a carbonaceous film has been proposed. In addition, as a method for manufacturing the electrode material, a method in which a slurry including the electrode active material or a precursor of the electrode active material and an organic compound is sprayed and dried so as to generate a granulated body, and the granulated body is thermally treated in a non-oxidative atmosphere of 500° C. or higher and 1,000° or lower has been proposed (for example, refer to Japanese Laid-open Patent Publication No. 2004-014340, Japanese Laid-open Patent Publication No. 2004-014341, and Japanese Laid-open Patent Publication No. 2001-015111).
In addition, a cathode material for a lithium-ion secondary battery in which the content of carbon in a complex of LiFePO4 and carbon is set in a range of 1 to 20% by mass (refer to Japanese Laid-open Patent Publication No. 2006-032241), a cathode active material for a lithium-ion secondary battery made of a lithium-containing phosphate agglomerate having an average particle diameter of 3 μm or less which is obtained by coating lithium-containing phosphate having an average particle diameter of 1 μm or less with a carbonaceous bonding agent and granulating the lithium-containing phosphate (refer to Japanese Laid-open Patent Publication No. 2009-048958), and the like have been proposed. In these cathode active materials, the density of the granulated body is improved, whereby it is possible to coat the cathode active material with a carbonaceous film in a uniform thickness and improve the battery characteristics. In addition, the density is improved, whereby the density of the cathode active material in electrodes can be increased, and the capacity can be increased, and furthermore, an increase in the density shortens the diffusion distance of lithium ions, enhances the diffusivity of lithium ions, and enables the improvement of ion conductivity in cathodes.